Wind loads in India: IS 875 Part 3 made simple

A solar array is a set of flat plates held up in the wind. On a stormy day, the wind can pull a module upwards with several times its own weight. Structures that fail in India almost always fail in wind: during cyclones on the east and west coasts, or in pre-monsoon storms inland. This lesson explains the Indian wind code in plain steps so you can see where the numbers come from. A structural engineer signs off the final design; your job is to understand it and install exactly what was designed.

Step 1: basic wind speed, Vb

IS 875 (Part 3): 2015 divides India into zones by basic wind speed (Vb): the 3-second gust speed at 10 m height in open terrain, with a 50-year return period. The zones are 33, 39, 44, 47, 50 and 55 m/s.

Values from Annex A as replaced by Amendment 2 (June 2020); several differ from older lists based on the 1987 code. Always confirm the site's value in the current Annex A and the wind map (now Fig. 1 of NBC 2016).

Step 2: design wind speed, Vz

Vz = Vb × k1 × k2 × k3 × k4

• k1, risk coefficient: 1.0 for normal structures with a 50-year design life.
• k2, terrain and height factor: wind is slower near rough ground and faster higher up. At 10 m height it is 1.05 for terrain category 1, 1.00 for category 2, 0.91 for category 3 and 0.80 for category 4. It rises with height; use the code's table for taller buildings.
• k3, topography factor: 1.0 on flat ground; higher on hilltops, ridges and cliff edges.
• k4, importance factor for cyclonic regions: 1.0 for ordinary structures; 1.15 for industrial structures and 1.30 for structures that must work after a cyclone (hospitals, emergency services) in the cyclone-prone coastal belt.

Terrain categories, simply:

Step 3: wind pressure, pz

pz = 0.6 × Vz² (pz in N/m², Vz in m/s)

Pressure goes with the square of speed. A wind 10% faster pushes 21% harder. That is why one zone step matters so much.

Step 4: design pressure, pd

pd = Kd × Ka × Kc × pz, but not less than 0.7 × pz

• Kd, directionality factor: 0.9 for buildings; take 1.0 in cyclone-affected regions.
• Ka, area averaging factor: 1.0 for small areas (10 m² or less), reducing for larger tributary areas.
• Kc, combination factor: 0.9 where it applies, otherwise 1.0.

Step 5: force on the array

F = Cf × A × pd

A is the module or array area and Cf is a force (or net pressure) coefficient taken from the code's tables for the shape: for an array on a roof, the tables for free-standing monopitch canopies or, where available, specific guidance for roof-mounted panels. Coefficients are higher at array edges and corners and change with tilt, wind direction and how blocked the space under the array is. We use Cf = 1.2 below purely to illustrate the arithmetic.

Wind from behind (from the north, under the raised back edge) gets under the module and lifts it, while suction on the top surface pulls it up. Wind from the front pushes the array down and back. The structure, the clamps and above all the anchors must take the uplift.

Worked example 1: a Pune bungalow

Two-storey house, array top below 10 m, suburban area.

1. Vb = 39 m/s
2. k1 = 1.0, k2 = 0.91 (category 3, 10 m), k3 = 1.0, k4 = 1.0 → Vz = 39 × 0.91 = 35.5 m/s
3. pz = 0.6 × 35.5² = 756 N/m²
4. pd = 0.9 × 1.0 × 0.9 × 756 = 612 N/m² (check: 0.7 × 756 = 529; 612 is higher, so 612 stands)
5. Per 550 Wp module (2.584 m²): F = 1.2 × 2.584 × 612 = 1,898 N ≈ 194 kg-force

The module weighs 28 kg. Wind can pull it upwards with about seven times its own weight. For 9 modules, about 17 kN (1.7 tonnes-force) of uplift, before edge effects.

Worked example 2: a sea-front house in Chennai

Two-storey house facing open sea, cyclone-prone coast.

1. Vb = 50 m/s
2. Terrain category 1 (open sea in front), k2 = 1.05, k4 = 1.0 (ordinary residence) → Vz = 50 × 1.05 = 52.5 m/s
3. pz = 0.6 × 52.5² = 1,654 N/m²
4. Cyclonic region, so Kd = 1.0: pd = 1.0 × 1.0 × 0.9 × 1,654 = 1,488 N/m²
5. Per module: F = 1.2 × 2.584 × 1,488 = 4,614 N ≈ 470 kg-force

That is 2.4 times the Pune figure for the same module. Many engineers also apply k4 = 1.15 to rooftop arrays in the 60 km cyclone belt as a prudent choice, which raises the pressure by a further 32%. Coastal sites need heavier sections, more anchors, lower tilt and height, and a stamped structural design.

What raises wind load on a rooftop array

• Height: taller buildings (k2 above 1.0) and walk-under structures that stand higher above the slab.
• Open terrain: sea-front, open fields, a house standing above its neighbours.
• Edges and corners of the roof: suction is strongest there. Keep arrays back from roof edges (the 0.6 m setback is a minimum; more near corners in high-wind zones).
• Higher tilt: a steeper module catches more wind and has a longer lever arm on the rear legs. This is one reason Indian rooftop designs cap tilt around 15–20°.
• Gaps under the array: an open, elevated array lets wind under it.

Parapets help by sheltering the array, but they also create turbulence at the edges. Follow the design, not instinct.

Key points

• Basic wind speed (33–55 m/s) comes from the IS 875-3 map for the site.
• Vz = Vb × k1 × k2 × k3 × k4; pz = 0.6 Vz²; pd = Kd × Ka × Kc × pz (not less than 0.7 pz).
• Pressure rises with the square of speed: Chennai's 50 m/s gives over twice Pune's pressure.
• Uplift on a module can be several times its weight; anchors and clamps must resist it.
• Height, open terrain, roof edges, higher tilt and cyclone exposure all increase wind load; coastal sites need a site-specific design.

Check yourself

1. Vz is 44 m/s. What is pz?
2. Why does Kd become 1.0 in cyclonic regions?
3. By how much does pressure rise if Vb goes from 39 to 47 m/s, with everything else equal?